Optical interconnects can offer significant advantages over electrical circuitry in the field of advanced microelectronics. One possible implementation of a deeply-scaled optical interconnect system is based on silicon-on-insulator (SOI) technology, in which optical waveguides are formed on the same thin silicon layer as other complimentary-metal-oxide-semiconductor (CMOS) circuit elements (e.g., field effect transistors (FETs), capacitors, resistors, etc.). Light sources produce optical signals (e.g., light pulses) that propagate in these optical waveguides. Photodetectors convert the optical signals into electrical signals.
A photodetector will typically need to be made from either silicon (Si) or germanium (Ge) in order to be compatible with CMOS processing. An implementation of a Ge photodetector is described in, for example, U.S. Patent Publication No. 2007/0189688 A1, entitled “Waveguide Photodetector,” which is commonly assigned herewith and is incorporated by reference herein. Embodiments of this invention describe a horizontal Ge waveguide photodetector that overlies and runs parallel with a horizontal Si waveguide. Light signals propagate in the Si waveguide and are simultaneously coupled into the Ge waveguide photodetector. Electrical contacts to the Ge waveguide photodetector allow the coupled light to be detected.
Nevertheless, the integration of Ge into a conventional CMOS process is complicated by the additional thermal budget required by Ge growth, the maximum temperature Ge can withstand, cross-contamination issues, Ge doping issues, Ge passivation issues, and the tendency of Ge to form non-ohmic contacts when mated with those metallic materials conventionally used for vertical contacts. There is a need, as a result, for structures and process integration schemes that overcome some or all of these issues and allow Ge waveguide photodetectors to be effectively fabricated in a manner that is compatible with conventional CMOS processing.